Injection molded plastic furniture includes chairs, tables, stools, plant stands, and many other useful forms of furniture. A major advantage of such furniture is its low manufacturing cost. Typically, such furniture is made of a thermoplastic such as polypropylene, polystyrene, polyethylene, acrylic, acrylonitrile butadiene styrene (ABS), or mixtures and combinations thereof. Fillers such as calcium or talc may also be added. The selection of which of the many commercially available plastics to use depends on a variety of design and production factors, principle among which are the strength, toughness, stiffness, and durability of the overall structure in view of the intended use of the furniture item. For economical reasons, often the furniture item is injected molded as a single piece or as a set of small number of pieces of the same plastic which are then assembled together. See for example, U.S. Pat. No. 7,401,854 B2 to Adams which discloses an injection molded stackable folding chair.
Sometimes it is useful or necessary to attach to the floor-contacting parts of the injected molded plastic furniture a separately manufactured foot. One benefit of using such a foot is to provide the article of furniture with improved friction in order to reduce the slippage of the article on smooth surfaces. Another is that, when feet are used on an article of furniture with legs such as a chair or table, the amount of internal stress the article must withstand when a load is applied is reduced. The internal stress reduction achieved by using feet can be very significant. The ASTM Test Results section later in this document illustrates differences in how long specific chairs hold a set weight before failing when feet are used and when they are not used. One chair held for 76 minutes with conventional feet, but only for about 1 minute with no feet. For that chair, and for many other articles of furniture, feet are a critical and integral component. Other reasons feet may be used are to cushion impacts on the furniture or to protect substrates from being scuffed by the more rigid material comprising the furniture.
Feet are usually attached to an article of furniture shortly after the injection molding of the furniture although they could also be attached sometime thereafter. The article of furniture is usually provided with a cavity or socket for receiving the anchor portion of the foot. With conventional feet, the cavity and the anchor portion are normally shaped and sized so that the foot is retained by friction. The anchor portion is designed to be slightly wider than the cavity and to be compressed into the cavity to create an interference fit (also known as a friction or press fit). In some cases, press fits create a satisfactory mechanical connection. However, they are not sufficient for connecting feet to furniture. The initial grip strength is on the low side. In addition, conventional feet are normally made of a semi-flexible material. Any semi-flexible material will take a compression set over time. So as time passes and the feet are compressed, the grip of the foot into the cavity lessens. Consequently, it is possible for frictionally retained feet to be jostled or knocked off (or to simply fall off) of the article of furniture to which they were attached. Although this conventional foot retaining method has been standard practice in the resin furniture industry for many years if not decades, it does not result in a reliable grip of the foot onto the chair. The ASTM Test Results section shows how if just one foot falls off it can cause a chair to no longer meet industry standards for outdoor furniture.
Even though semi-flexible materials take a compression set, they do not take a set as quickly as fully flexible, softer, lower-durometer materials. That is why semi-flexible materials are used for furniture feet instead of a softer material. The trade-off of not using softer foot material is that some grip on surfaces is sacrificed. Softer feet would provide more frictional grip on substrates.
The last weakness of conventional feet is the difficulty of inserting them. Since the anchor portion of the foot must be made wider than the cavity to create a friction fit, assemblers must exert themselves to squeeze the foot into the cavity. Often feet are inserted only to the point where they are stable enough to stay in place until they can be hammered fully in. Still, getting the feet even partially inserted into the cavity is difficult with conventional feet.
Another type of foot commonly used for furniture consists of a rubber washer with a bolt that passes through the center of the washer. The washer may be seated in a cylindrical metal housing to which a bolt or threaded rod is attached. Usually the head of the bolt is recessed into the washer so that only the washer makes contact with the floor. An example of such a product is available from Custom Rubber Corp., and sold as a Non-Marking Molded Rubber Leveling Foot. The leg in which the foot is attached typically has a threaded cavity or nut into which the bolt is secured. While this type of foot is securely held, several minutes may be required to install these feet on the legs of three-legged or four-legged furniture. The feet themselves are also much more expensive than feet which are extruded and friction fitted into a leg cavity.
It is also known to provide rubber caps encompassing the head of a bolt; such as the Molded Rubber Bumper Bolts again made by Customer Rubber Corp. With this type of foot, there are multiple issues. First among them, the rubber must be made very hard so that it cannot slip away from the head of the bolt since soft, flexible plastic would not have adequate resistance to decoupling from the head of the bolt during use. Also, the assembly of such feet would be time consuming and/or require special receiving cavities in the furniture. Next, the cost of such feet, due to the need for a somewhat large metal bolt as a component and to the expensive nature of insert molding, is much higher than with conventional feet. Finally, such feet could come partially or fully unscrewed during use.